1. Field of the Invention
The present invention relates to an image forming apparatus, and, more particularly, to an image forming apparatus capable of detecting a paper level in a paper supply tray.
2. Description of the Related Art
An image forming apparatus, such as an electrophotographic printer, may include a supply tray which holds print media, such as paper. The media is held in the supply tray until a print job is requested, and is transported to an electrophotographic (EP) assembly within the printer where a latent image is transferred thereto. The media sheets are usually intended to be transported one by one from the supply tray and through a paper path to the EP assembly.
A supply tray of an image-forming apparatus can be variously configured. For example, one known configuration includes a supply tray having a bottom plate which is spring loaded from the bottom and biased in an upwardly direction. The spring loaded bottom plate biases a paper stack disposed within the supply tray in an upwardly direction against a comer buckler. A picker assembly, which may include a pick roller, engages the top of the top sheet of the paper stack and moves the top sheet into the paper path.
Another type of known supply tray includes a ramped surface or dam at an end thereof which is adjacent to the paper path in the printer. The paper in the supply tray is not biased in an upwardly direction, but rather merely lays on the bottom of the supply tray. A picker assembly includes a picker which engages the top of the top sheet in the paper stack and moves the top sheet up the dam and into the paper path of the printer.
A user of an image forming apparatus including a supply tray may find it desirable to be alerted as to the status of the number of media sheets (or paper level) within the supply tray either before or during a print job. For example, if a supply tray has a capacity of 200 sheets, it may be desirable for a user to be aware that the supply tray is approximately half full, or has approximately 100 sheets therein. Further, it may be desirable for a user to be alerted that the supply tray is nearly empty so that it may be replenished prior to a print job. Moreover, it may be desirable for a user to know that the number of printed pages in a requested print job is larger than the approximate number of sheets in the supply tray.
Various methods and apparatus exist for determining the approximate paper level within a supply tray. All such conventional paper level indicators require relatively expensive additional hardware to be added to the printer to sense the paper level, thereby increasing the cost of the printer. Generally, these methods and apparatus are used in conjunction with a supply tray wherein the paper rests directly on the bottom of the supply tray. For example, one method uses an arm which rests on the top of the paper stack and uses hardware to detect the angle of the arm. Such methods for determining a paper level in a supply tray utilize an arm which directly rests on top of a paper stack in the supply tray.
Further, it is also known to provide a paper level indicator wherein a stack of paper is disposed within a feeder module such that the trailing edges of the paper sheets are disposed at an acute angle relative to a bottom surface of the feeder module. A light source is disposed on top of the feeder module and a sensor is disposed on the bottom of the feeder module. As the sheets in the paper stack are used, more of the light which is output by the light source is received by the sensor. The sensor operates a suitable electrical circuit providing a "low" warning to a user when the height of the stack is such that the sensor is substantially exposed. Such an apparatus is disclosed in U.S. Pat. No. 4,928,949 (Ramsey, et al). Such an apparatus does not utilize an arm which rests upon the top of the paper stack within the supply tray, but still requires the use of multiple coacting sensors, thereby adding to the cost of the printer.
What is needed in the art is an apparatus for detecting a paper level in a supply tray which does not require a substantial amount of additional hardware.